About 13.7 percent of Florida births occurred before the mothers' 37th week of pregnancy, putting those babies at significantly higher risk of disability or death, the report says. That rate is up less than a full percentage point from 2011, but the increase still troubles experts who have worked years to address it.

The national rate dropped to 11.5 percent, a 15-year low.

Florida was one of five "D" states,mostly in the South, on the March of Dimes report card. Only three states — Alabama, Mississippi and Louisiana — earned an "F" grade. Six states earned an "A."

Experts consider a baby full-term after 39 to 41 weeks.

The reasons behind Florida's increase are unclear and could be just a one-year blip. But public health experts say they believe there's plenty more a D-state like Florida can do reduce the number of babies born too early.

One of the March of Dimes' recommendations? Make more adults eligible for Medicaid, a key portion of the federal health care law rejected by legislatures in nearly half the states, including Florida.

The Medicaid program already covers many women during their pregnancy. But research shows that many risks of early delivery — diabetes or high blood pressure in the mother, for instance — originate before conception, said Dr. William Sappenfield, director of the Chiles Center for Healthy Mothers and Babies at the University of South Florida.

"The idea of 'I'm going to just take care of you only when you're pregnant' implies I can take care of all your problems while you're pregnant," he said. "I'm not trying to be radical. I'm just talking about people getting basic health care."

Florida's Healthy Start Coalitions coordinate care for pregnant women, including helping them quit smoking, one of the major risk factors for premature births, said Evie Fox, president of the Florida Association of Healthy Coalitions. Since Healthy Start began in 1993, she said, the infant mortality rate — defined as babies who do not survive their first year — has declined from 8.6 percent to 6 percent.

"It is counterintuitive that this type of assistance to moms and babies would not lead to better outcomes," Fox said.

Sappenfield said one reason more babies are surviving is because the rate of prematurity has generally declined in recent years — progress that appears to have slightly reversed last year. He said education in better health habits, like medical care, can do more to help prevent early births if started before conception. Getting a woman to quit smoking before she gets pregnant, for instance, will increase her chances of having a healthy baby.

March of Dimes officials noted that one project to reduce early births has had limited success. As part of a national March of Dimes initiative, several hospitals around the state, including St. Joseph's Hospital in Tampa, allow early deliveries only for medical necessity, not convenience.

The initiative successfully reduced the number of deliveries in the 37th and 38th weeks of pregnancies, but had no effect on earlier deliveries.

Preterm births cost the nation more than $26 billion annually, according to the Institute of Medicine. It is the leading cause of newborn death, and babies who survive an early birth often face the risk of serious and sometimes lifelong health issues, such as breathing problems, jaundice, developmental delays, vision loss and cerebral palsy.

"This year's increase in our preterm birth rate is disappointing," Dr. Karen Harris, the chairwoman of March of Dimes Florida's program services, said in a statement. "We expect this to be a one-year setback in our long-term progress toward preventing premature birth and giving more babies a healthy start in life."

Jodie Tillman can be reached at jtillman@tampabay.com or (813) 226-3374.